require(datasets)
class(iris) 
class(iris3) 
mean.iris=by(iris[,1:4],iris$Species,colMeans) 
dim(iris3)
mean.iris3=apply(iris3,MARGIN=c(2,3),FUN=mean) 
class(mean.iris3)
nomes=c("Comp sepula","Larg sepula","Comp petala","Larg petala") 
rownames(mean.iris3)<-c(nomes)